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Effects of Elevated CO2 and Light on Four Weedy Species
Leishman, M.R., Sanbrooke, K.J. and Woodfin, R.M.  1999.  The effects of elevated CO2 and light environment on growth and reproductive performance of four annual species.  New Phytologist 144: 455-462.

What was done
Four weedy C3 plants, common to European grasslands, were grown from seed to senescence in glasshouses receiving atmospheric CO2 concentrations of 350 and 550 ppm.  In addition, these annual species (Cardamine hirsute, Spergula arvensis, Senecio vulgaris, and Poa annua) were grown at full light and reduced light (67% of full light) levels, to determine the interactive effects of elevated CO2 and light on their vegetative and reproductive success.

What was learned
Elevated CO2 did not significantly impact vegetative growth in three of these weedy species.  For Spergula arvensis, however, atmospheric CO2 enrichment increased maximum leaf length by an average of 15%, regardless of light treatment, and total dry weight by 20 and 68% at full and reduced light levels, respectively.  Similarly, elevated CO2 significantly enhanced reproductive success in only one of these species, by increasing the number of seeds in Poa annua by 50 and 26% at full and reduced light levels, respectively.  Thus, CO2-induced increases in vegetative biomass do not necessarily lead to increases in reproductive biomass, and CO2-induced increases in reproductive biomass do not need to be preceded by increases in vegetative biomass.

In contrast to the limited significant effects of elevated CO2 on vegetative and reproductive growth, reduced light nearly always caused significant decreases in plant vegetative and reproductive success.  Moreover, the negative effects of reduced light on these parameters were typically much stronger than any positive effect resulting from atmospheric CO2 enrichment.  Total plant dry weights, for example, decreased by 32 to 62%, depending upon species, when grown in reduced, rather than in full light, while atmospheric CO2 enrichment only increased total plant dry weights by 2 to 30%.

What it means
As the CO2 content of the air continues to rise, it is likely that these four weedy species common to European grasslands will not flourish and expand their territories as often claimed by some individuals who propagate the view that rising CO2 levels will cause weeds to become increasingly more problematic and devastating to ecosystems in the future.  Rather, it is more conceivable that a single weedy species, Poa annua, may possess the ability to expand its ranges, but only in habitats receiving full sunlight; for this paper demonstrated that light level was a much greater determinant of vegetative and reproductive success than was atmospheric CO2 enrichment.  Thus, if the results of this paper are applicable to most weedy species, it is likely that they will exhibit few, if any, changes in their vegetative and reproductive success in response to future increases in the CO2 content of the air.

Reviewed 15 May 2000